In general, a suspension apparatus of a vehicle includes a shock absorber and a spring member which are disposed between sprung and unsprung members of the vehicle. The spring member generates an elastic force, and the shock absorber generates a damping force. The damping force damps vibrations between the sprung member (a member mounted on the vehicle body side) and the unsprung member (a member mounted on the wheel side) which are caused by approaching and separating motions between the sprung member and the unsprung member.
There has been known a shock absorber in which a motor is used. The motor of this shock absorber includes a stator connected to one of the sprung and unsprung members, and a rotor which rotates in relation to the stator. This shock absorber may have a motion conversion mechanism which converts the approaching and separating motions between the sprung and unsprung members to a rotational motion and transmits the resultant rotational motion to the rotor.
According to this shock absorber, when the sprung and unsprung members approach and separate from each other, the rotor of the motor is caused to rotate in relation to the stator via, for example, the above-described motion conversion mechanism. An inductive electromotive force (counter electromotive force) is generated as a result of rotation of the rotor. An induced current (generated current) flows through the motor as a result of generation of the inductive electromotive force. As a result of the generated current flowing through the motor, there is generated a motor torque which acts in a direction opposite the rotation direction of the rotor. Such motor torque is used as the damping force.
Japanese Patent Application Laid-Open (kokai) No. 2009-257486 discloses a vibration system which includes a main vibration system and an additional vibration system added thereto, wherein the main vibration system and the additional vibration system vibrate in a direction in which they approach and separate from each other. The additional vibration system includes a motor and a ball screw mechanism (motion conversion mechanism). In addition, an RLC series circuit is connected to the motor for causing the generated current to flow through the motor. By use of the RLC series circuit, the inertial force, viscous force, and elastic force of this vibration system are represented.